Air distribution method and controller for a fuel cell system

ABSTRACT

An airflow control system and method for a fuel cell includes a compressor that supplies air to a storage chamber for storing the air. Fuel cell subsystems are connected to the air storage chamber. Each of the fuel cell subsystems includes a flow controller and flow sensor. A sensor measures air pressure in the storage chamber. A controller polls the flow controllers of the fuel cell subsystems for a minimum required air pressure for the fuel cell subsystems. The controller selects a highest minimum required air pressure. The controller controls the compressor to provide the highest minimum required pressure in the air storage chamber. The air storage chamber includes tubing, a manifold or both.

FIELD OF THE INVENTION

[0001] The present invention relates to fuel cells, and moreparticularly to the distribution of air in a fuel cell system.

BACKGROUND OF THE INVENTION

[0002] Fuel cell systems are increasingly being used as a power sourcein a wide variety of applications. Fuel cell systems have also beenproposed for use in vehicles as a replacement for internal combustionengines. The fuel cells generate electricity that is used to chargebatteries or to power an electric motor. A solid-polymer-electrolytefuel cell includes a membrane that is sandwiched between an anode and acathode. To produce electricity through an electrochemical reaction,hydrogen (H₂) is supplied to the anode and oxygen (O₂) is supplied tothe cathode. In some systems, the source of the hydrogen is reformateand the source of the oxygen (O₂) is air.

[0003] In a first half-cell reaction, dissociation of the hydrogen (H₂)at the anode generates hydrogen protons (H⁺) and electrons (e⁻). Themembrane is proton conductive and dielectric. As a result, the protonsare transported through the membrane while the electrons flow through anelectrical load (such as the batteries or the motor) that is connectedacross the membrane. In a second half-cell reaction, oxygen (O₂) at thecathode reacts with protons (H⁺), and electrons (e⁻) are taken up toform water (H₂O).

[0004] There are several fuel cell subsystems within a fuel cell systemthat require a separately controlled source of pressurized air. Forexample, these fuel cell subsystems include combustors, partialoxidation (POx) reactors, preferential oxidation (PrOx) reactors, thefuel cell stack and/or other fuel cell subsystems. The fuel cellsubsystems typically employ mass flow controllers, mass flow sensors andone or more compressors to provide the air.

[0005] When two or more fuel cell subsystems require a controlled amountof pressurized air, some conventional fuel cell systems use a compressorfor each subsystem. Each compressor is typically controlled based on thedesired airflow that is required by the associated fuel cell subsystem.While this control method is accurate and relatively simple from acontrol standpoint, the duplication of compressors is undesirable fromcost, weight and packaging standpoints.

[0006] In other conventional fuel cell systems, a single compressorsupplies the air to all of the fuel cell subsystems. A controller sumsthe mass flow requirements for all of the fuel cell subsystems. Thecontroller commands the compressor to provide the summed mass flowrequirement. In this fuel cell control system, an overflow valve istypically required to bleed off excess air due to system errors. Thetransient response of this control method is inherently compromised dueto coupling between the fuel cell subsystems. This control system alsorequires significant rework for any changes in the fuel cell system.

[0007] For example, when mass flow-based control is used and five fuelcell subsystems request 1 g/s flow, the controller sums the mass flowrates and attempts to provide 5 g/s. If one of the flow sensors isinaccurate, all of the fuel cell subsystems suffer. If one of the fuelcell subsystems has a faulty mass flow sensor or mass flow controllerand the fuel cell subsystem actually achieves 1.5 g/s but requires 1g/s, each of the other fuel cell subsystems are starved of air.Alternately, if the faulty fuel cell subsystem requests 2 g/s but getsonly 1 g/s, all of the other fuel cell subsystems receive too much air.In other words, an error in one fuel cell subsystem causes errors in thedelivery of air to all of the other fuel cell subsystems.

SUMMARY OF THE INVENTION

[0008] An airflow control system and method for a fuel cell according tothe invention includes a compressor that supplies air to a storagechamber. Fuel cell subsystems are connected to the air storage chamber.A sensor measures air pressure in the storage chamber. A controllerpolls the fuel cell subsystems for a minimum required air pressure. Thecontroller selects a highest minimum required air pressure. Thecontroller controls the compressor to provide the highest minimumrequired pressure in the storage chamber.

[0009] In other features of the invention, the storage chamber includestubing or a manifold or both. Each of the fuel cell subsystems includesa flow controller and flow sensor. The controller periodically polls thefuel cell subsystems for the minimum required air pressure. The flowcontroller preferably includes an electronic throttle valve. The flowsensor preferably includes a hot wire anemometer.

[0010] In other features of the invention, the fuel cell subsystems areselected from the group of combustors, partial oxidation (POx) reactors,preferential oxidation (PrOx) reactors, fuel cell stacks, a cathodeinlet of a fuel cell stack, and an anode inlet of a fuel cell stack.

[0011] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0013]FIG. 1 is a schematic block diagram illustrating an airflowcontrol system according to the prior art;

[0014]FIG. 2 is a simplified mass airflow-based control diagram inaccordance with the prior art;

[0015]FIG. 3 is a schematic block diagram illustrating an airflowcontrol system according to the present invention;

[0016]FIG. 4 is a pressure-based airflow control diagram according tothe present invention; and

[0017]FIG. 5 is a flowchart illustrating steps for controlling thecompressor according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The following description of the preferred embodiment(s) ismerely exemplary in nature and is in no way intended to limit theinvention, its application,or uses.

[0019] Referring now to FIG. 1, an air delivery system 10 for a fuelcell system 12 is illustrated. The fuel cell system 12 includes aplurality of fuel cell subsystems 14-1, 14-2, . . . 14-n that requirethe controlled delivery of air. For example, the fuel cell subsystem14-1 includes a mass airflow sensor 16-1, a mass airflow controller18-1, and a combustor 20. The mass airflow sensor 16-1 measures the massairflow of air flowing through the tubing 22-1. The mass airflowcontroller 18-1 adjusts and controls the mass airflow to the combustor20. As can be appreciated, the mass flow controller 18-1 may beconnected to one or more controllers that are associated with thecombustor 20 or other fuel cell subsystems.

[0020] The other fuel cell subsystems 14-2, 14-3, . . . , 14-n likewisecontrol the airflow to other fuel cell components. For example, the POxreactor 24 partially oxidizes the supply fuel to carbon monoxide andhydrogen (rather than fully oxidizing the fuel to carbon dioxide andwater). Air and fuel stream are injected into the POx reactor 24. Theadvantage of POx over steam reforming of the fuel is that it is anexothermic reaction rather than an endothermic reaction. Therefore, thePOx reaction generates its own heat. The mass airflow sensor 16-2 sensesthe airflow in the tubing 22-2. The mass airflow controller 18-2 adjustsand controls the airflow that is delivered to the POx reactor 24. Themass airflow controller 18-2 may be connected with one or morecontrollers that are associated with the POx reactor 24 or other fuelcell subsystems.

[0021] Similarly, mass airflow sensors 16-3, 16-4, 16-5, . . . , 16-nsense airflow in tubing 22-3, 22-4, 22-5, . . . , 22-n. Mass flowcontrollers 18-3, 18-4, 18-5, . . . 18-n adjust and control the airflowthat is delivered to a preferential oxidation (PrOx) reactor 26, ananode input 30 of a fuel cell stack 31, a cathode input 32 of the fuelcell stack 31, and any other fuel cell subsystems 36 that require airinput.

[0022] The air is typically supplied by a compressor 37. A cooler 38cools the air that is output by the compressor 37 to a manifold 40and/or to the tubing 22. A mass flow sensor 42 senses the airflow thatis produced by the compressor 37. An airflow controller 50 is connectedto the mass airflow sensors 16 and 40, the mass airflow controllers 18,and the compressor 37. The airflow controller 50 sums the airflowrequirements of each of the fuel cell subsystems 14 that require airinput. The airflow controller 50 adjusts and controls the mass airflowof the compressor 36 to meet the summed airflow demand of the fuel cellsubsystems 14.

[0023] Referring now to FIG. 2, the control strategy of the massflow-based airflow controller 50 is illustrated and is generallydesignated 100. The desired mass flow rate for first, second, . . . ,and n^(th) fuel cell subsystems 102, 104, and 106 are summed by a summer110 to generate a target mass flow rate 112 for the compressor 37. Theairflow controller 50 commands the compressor 37 to provide the targetmass flow rate 112. In this control system, an overflow valve istypically required to bleed off excess air pressure that accumulates dueto system errors. The transient response of this control method iscompromised due to the coupling between the fuel cell subsystems. Inother words, a control error in one fuel cell subsystem adverselyimpacts all of the fuel cell subsystems. This control system alsorequires significant rework for any changes in the fuel cell subsystems.

[0024] Referring now to FIG. 3, a pressure-based airflow control system120 is illustrated. For purposes of clarity, reference numerals fromFIG. 1 have been used where appropriate to identify the same elements.The pressure-based airflow control system 120 includes a pressure sensor122 that measures air pressure in the manifold 40 and/or the tubing 22.The airflow controller 50 periodically polls the fuel cell subsystems 14and requests the minimum air pressure that is required by each of thefuel cell subsystem 14. The fuel cell subsystems 14 provide the minimumrequired pressure. If no pressure is required, then the fuel cellsubsystems 14 do not respond or respond with zero. One or more of thefuel cell subsystems 14 may have no pressure requirement during a givenpolling period. The airflow controller 50 selects the highest minimumpressure from the minimum required pressures output by the fuel cellsubsystems 14. The airflow controller 50 controls the air pressure inthe manifold 40 and/or tubing 22 to maintain the highest minimumrequired pressure for the fuel cell subsystems 14 until the subsequentpolling period.

[0025] Referring now to FIG. 3, the control strategy employed by theairflow controller 50 in the pressure-based airflow control system 120is shown in further detail. The airflow controller 50 monitors thepressure P of air in the manifold 40 and/or the tubing 22. The airflowcontroller 50 polls the fuel cell subsystems 14 for their highestminimum pressure. The airflow controller 50 selects the highest minimumrequired pressure P_(min). The airflow controller 50 compares themonitored pressure P in the manifold 40 to the highest minimum requiredpressure P_(min).

[0026] An actual pressure signal 206 that is generated by the pressuresensor 122 is input to an inverting input of the summer 204. The highestminimum required pressure P_(min) 202 is input to a non-inverting inputof the summer 204. An output of the summer 204 is input to one or moregain blocks 210 and 212. The gain block 210 provides a system pressuregain. The gain block 212 represents other required fuel cell systemgains. An output of the gain block 212 is input to a summer 216. Anactual or estimated compressor mass flow rate 218 is input to the summer216. The compressor mass flow rate 218 can be estimated from the speedof the compressor 37 and the inlet and outlet pressure of the compressor37. An output 220 of the summer 216 is equal to the target mass flowrate for the compressor 36.

[0027] Referring now to FIG. 5, steps for controlling the pressure-basedairflow control system 120 are shown in further detail and are generallydesignated 250. Control begins with step 252. In step 253, a pollingtimer that is associated with the airflow controller 124 is reset. Instep 254, the airflow controller 124 polls the fuel cell subsystems 14for their minimum pressure requirement. In step 256, the airflowcontroller 124 selects the highest minimum pressure P_(min) that isrequired by the fuel cell subsystems 14. In step 258, the airflowcontroller 124 measures the pressure P in the manifold 40 and/or in thetubing 22. In step 262, the airflow controller 124 determines whetherthe polling timer is up. If it is, control continues with step 253.Otherwise, control continues with step 266. In step 266, the airflowcontroller 124 determines whether the measured pressure P exceeds thehighest minimum pressure P_(min). If the measured pressure P exceeds thehighest minimum pressure P_(min), then control continues with step 262.If the measured pressure P does not exceed the highest minimum pressureP_(min), control continues with step 270. In step 270, the pressure P inthe manifold 40 and/or the tubing 22 is increased using the compressor36.

[0028] In the present invention, the fuel cell subsystem airflowdynamics are directly proportional to the pressure in the manifoldand/or the tubing 22 and are not directly related to the mass flow rateof the compressor 37. The mass flow rate of the compressor 37 indirectlyaffects the dynamics of the fuel cell subsystems 14 by affecting therate of change of the pressure P in the manifold 40 and/or the tubing22. The airflow controller 124 provides much tighter transient controlof the airflow to the fuel cell subsystems. In addition, the airflowcontroller 124 de-couples the interactions between the fuel cellsubsystems to a larger extent than conventional airflow controllers. Asa result, the downstream fuel cell subsystems can be more efficientlydeveloped in a distributed manner.

[0029] The airflow controller 124 has improved disturbance rejection ascompared to conventional airflow controllers. In addition, the massairflow sensor that measures compressor airflow can be eliminated toreduce cost due to the lower coupling of the pressure of the pressurebased control strategy. The mass flow rate of the compressor 37 can beestimated from the speed and input and output pressures of thecompressor 37. The overflow valve or pressure regulator can also beeliminated. The airflow controller according to the present inventionrequires a single compressor to control the airflow to multiple fuelcell subsystems, which improves cost, complexity, weight and packaging.The airflow controller also supports distributed development of the fuelcell subsystems, simplifies the development process by decoupling thefuel cell subsystems, and increases the potential for modularity.

[0030] Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

What is claimed is:
 1. An airflow control system for a fuel cellcomprising: an air supplier for supplying air; a volume for storing saidair; a plurality of fuel cell subsystems connected to said volume; asensor for sensing air pressure in said volume; and a controller thatreceives a minimum required air pressure for each of said fuel cellsubsystems.
 2. The airflow control system of claim 1 wherein saidcontroller selects a highest minimum required air pressure and controlssaid air supplier to provide said highest minimum required pressure insaid volume.
 3. The airflow control system of claim 1 wherein said airsupplier includes a compressor.
 4. The airflow control system of claim 1wherein said volume includes tubing.
 5. The airflow control system ofclaim 1 wherein said volume includes a manifold.
 6. The airflow controlsystem of claim 1 wherein said volume includes a manifold connected totubing.
 7. The airflow control system of claim 1 wherein said controllerperiodically polls each of said fuel cell subsystems for said minimumrequired air pressure.
 8. The airflow control system of claim 1 whereinsaid fuel cell subsystems include a flow controller and a flow sensor.9. The airflow control system of claim 8 wherein said flow controllerincludes an electronic throttle valve and said flow sensor includes ahot wire anemometer.
 10. The airflow control system of claim 1 whereinsaid fuel cell subsystems include a component that is selected from thegroup of combustors, partial oxidation reformer, preferential oxidationreactor, fuel cell stacks, a cathode inlet of a fuel cell stack, and ananode inlet of a fuel cell stack.
 11. The airflow control system ofclaim 1 wherein each fuel cell subsystem includes a flow controller andsaid controller polls said flow controller for said minimum required airpressure of said fuel cell subsystem.
 12. A method for controllingairflow to fuel cell subsystems in a fuel cell, comprising the steps of:supplying air to an air storage chamber; connecting a plurality of fuelcell subsystems to said air storage chamber; sensing air pressure insaid air storage chamber; and polling each of said fuel cell subsystemsfor a minimum required air pressure.
 13. The method of claim 12 furthercomprising the steps of: selecting a highest minimum required airpressure; and maintaining said highest minimum required air pressure insaid air storage chamber.
 14. The method of claim 12 wherein said air isprovided by a compressor.
 15. The method of claim 12 wherein said airstorage chamber includes tubing.
 16. The method of claim 12 wherein saidair storage chamber includes a manifold.
 17. The method of claim 12wherein said air storage chamber includes a manifold connected totubing.
 18. The method of claim 12 further comprising the step ofperiodically polling said fuel cell subsystems for said minimum requiredair pressure.
 19. The method of claim 12 wherein said fuel cellsubsystems include a flow controller and a flow sensor.
 20. The methodof claim 19 wherein said flow controller includes an electronic throttlevalve and said flow sensor includes a wire manometer.
 21. The method ofclaim 12 wherein said fuel cell subsystems include a component that isselected from the group of combustors, partial oxidation reformer,preferential oxidation reactor, fuel cell stacks, a cathode inlet of afuel cell stack, and an anode inlet of a fuel cell stack.
 22. An airflowcontrol system for a fuel cell comprising: a compressor that suppliesair; a volume for storing said air; a plurality of fuel cell subsystemsconnected to said volume, wherein each of said fuel cell subsystemsinclude a flow controller and flow sensor; a sensor for sensing airpressure in said volume; and a controller that polls said flowcontrollers of said fuel cell subsystems for a minimum required airpressure for said fuel cell subsystems, that selects a highest minimumrequired air pressure, and that controls said compressor to provide saidhighest minimum required pressure in said volume.
 23. The airflowcontrol system of claim 22 wherein said volume includes tubing.
 24. Theairflow control system of claim 22 wherein said volume includes amanifold.
 25. The airflow control system of claim 22 wherein said volumeincludes a manifold connected to tubing.
 26. The airflow control systemof claim 22 wherein said controller periodically polls said fuel cellsubsystems.
 27. The airflow control system of claim 22 wherein said flowcontroller includes an electronic throttle valve and said flow sensorincludes a wire manometer.
 28. The airflow control system of claim 22wherein said fuel cell subsystems include a component that is selectedfrom the group of combustors, partial oxidation reformer, preferentialoxidation reactor, fuel cell stacks, a cathode inlet of a fuel cellstack, and an anode inlet of a fuel cell stack.